Chlorine dioxide (ClO2) is known to be extremely effective for use as an antimicrobial, disinfectant, deodorizer, sterilizer, sanitizer, fungicide, germicide, and so forth. One problem associated with chlorine dioxide, however, is that it exists in a gaseous state, and as such is difficult to transport commercially. Chlorine dioxide as a concentrated gas is explosive and poisonous.
One common method for using or incorporating chlorine dioxide gas has been to dissolve the gas in a liquid to form a solution and attempt to stabilize the dispersed gas using chemical adjuvants such as polyvinyl pyrrolidone, metal complexes, inorganic salts, viscosifiers, and so forth. These methods have a number of drawbacks. The most common problem being that the chlorine dioxide gas tends to release from the solution so that its shelf-life is relatively short. Since some applications require hours, days, or even weeks of solution use time for the chlorine dioxide formulation, a strict regimen of gaseous reapplication would be necessary to ensure adequate chlorine dioxide concentrations over time, even with all the currently known solution stabilizing additives.
As a consequence, the common practice has evolved to generating chlorine dioxide right at the site where it is being used. Such generation methods are outlined in Chlorine Dioxide, Chemistry and Environmental Impact of Oxychlorine Compounds; Masschelein, W. J., Ann Arbor Science Inc., 1979, and typically employ the use of chlorine dioxide generating or liberating compounds such as chloric acid, chlorites and chlorates in applications in which chlorine dioxide is being used as a disinfectant, sterilizer, deodorizer, sanitizer, antiseptic, fungicide, germicide, and so forth.
The generation of chlorine dioxide from sodium chlorite or some other chlorine dioxide liberating compound can be broadly classified into three categories including the acidification of chlorites, the oxidation of chlorites, or the reduction of chlorites. Chlorine dioxide generation is thus usually activated by the addition of an acid, the addition of an oxidant like bleach (i.e. hypochlorite or hypochlorous acid), persulfate, or chlorine, or the addition of a reductant to chlorates (chemical or electrochemical).
Typically, however, the generation of chlorine dioxide has been accomplished either in the laboratory or at industrial levels at low pH values of 3 or less. Compositions that have a low pH are a problem for application to the skin of humans or animals such as teat dips because such acidity can cause skin irritation and burning. Additionally, acidic compositions can be corrosive to materials used in industrial equipment including metals, elastomers, plastics, cements and concretes, woven materials, and so forth. Problems due to corrosion of equipment can obviously have a negative economical impact when chlorine dioxide is used as a sanitizer for industrial equipment. Raising the pH to levels at which there is no skin irritation results in compositions that generate chlorine dioxide at an undesirably slow rate, or favors secondary reaction routes which simultaneously produce other more undesirable chlorine species (e.g., chlorates, chlorides, chlorine gas, and so forth). For example, an equimolar mixture of potassium chlorate and hydrochloric acid yields chlorine dioxide to chlorine gas at a ratio of about 1.0:1.35.
Patents relating to acid catalysis include U.S. RE36064, U.S. Pat. No. 4,585,481, U.S. Pat. No. 5,165,910, U.S. Pat. No. 5,651,996 and U.S. Pat. No. 5,853,689. As noted above, and reviewed in chapter 13 of Masschelein, W. J., the general rule is that the stronger the acid, the faster and more efficient the production of chlorine dioxide. For industrial applications, hydrochloric, sulfuric, or acetic are the most widely used acids, and the rate of chlorine dioxide generation and the overall yield of chlorine dioxide are improved using an excess of the acid (often up to 2–3 times excess). However, the lower the pH, the more corrosive the composition to equipment or treated surfaces, and the more irritation and burning to the skin of humans or animals. Comparatively, if the acid concentrations are reduced to concentrations that are too low, the rate of generation and the overall yield of chlorine dioxide are dramatically reduced to unusable levels.
A patent relating to the reduction of chlorates is U.S. Pat. No. 5,382,520. A patent relating to the oxidation of chlorites is U.S. Pat. No. 5,227,306 which describes a chlorite-chlorine system.
U.S. Pat. No. 6,231,830 describes a method for manufacturing molecular chlorine dioxide, by the addition of potassium iodide to a solution of alkali metal chlorite.
Some further disadvantages to previously used methods of chlorine dioxide generation include the formation of undesirable secondary by-products such as chloride or chlorine, high equipment costs due to the complexity of the equipment required, and the potential of explosion from localized heat development or chlorine dioxide head-space gas development. Disadvantages also result from the handling, dispensing and regulation of poisonous chlorine gas.
Some antimicrobial agents which are lethal to microorganisms can also be toxic in varying degrees to humans and animals, in that both higher and lower forms of life share at least some common metabolic pathways.
There remains a need for an effective means of generating chlorine dioxide in situ at pH values where skin irritation is not a problem, and the rate of chlorine dioxide generated is rapid, and the yield potentially long lasting. Furthermore, there has been a long felt need for antimicrobial agents which have a high degree of antimicrobial efficacy, and which can be safely used, and pose no environmental incompatibility.